1. Field of the Invention
The present invention relates to a tactile push button switch and, more particularly, to a drip-proof structure for such a switch.
2. Description of the Prior Art
Referring first to FIGS. 1, 2 and 3, a prior art example will be described.
A tactile push button switch 20 has a switch case 11 and a switch cover 12. The switch case 11 has at its center a first fixed contact 31 formed integrally therewith, from which terminals 34a and 34b lead off for external connection use. Similarly formed integrally in the switch case 11 are two second fixed contacts 32 and 33, from which terminals 35 and 36 (hidden behind terminals 34a and 34b, respectively) lead off for external connection use. A dome-shaped reversing contact 14 is formed of a resilient spring material such as phosphor bronze and has a marginal portion 42 rested on the bottom of the case 11 and held in contact with the second fixed contacts 32 and 33 exposed on the bottom of the case 11. Reference numeral 41 denotes the top or vertex of the dome-shaped reversing contact 14. Upon depression of a push button 15, the dome-shaped reversing contact 14 is pressed downward by a protrusion 52 of the push button 15, and the contact 14 rapidly reverses into top down position with a click the instant its vertex 41 reaches a dead center. The push button 15 has the protrusion 52 protrusively provided on the underside of the push button body and a flange 53 extended about its periphery. The push button 15 slightly projects upwardly of the surface of the switch cover 12 through an opening 12A bored therethrough. The switch case 11 and the switch cover 12 are ultimately molded into a watertight unitary structure.
In the normal condition with the push button 15 left unpressed, the dome-shaped reversing contact 14 retains its original shape by virtue of its own resiliency, and hence it pushes upward the push button 15 through its protrusion 52, urging its flange 53 against the underside of the switch cover 12 and holding the push button 15 at its raised position. In this instance, the dome-shaped reversing contact 14 is not in contact with the first fixed contact 31, holding the switch open. In other words, the terminals 34a, 34b and the terminals 35, 36 are electrically disconnected and no current flows therethrough.
Upon depression of the push button 15, the dome-shaped reversing contact 14 is pressed down and deformed by the protrusion 52, and the instant the vertex 41 of the dome reaches dead center, the contact 14 reverses into a top down position, bringing the vertex 41 into contact with the first fixed contact 31 centrally thereof as indicated by 30. As the result, the first fixed contact 31 is electrically connected through the dome-shaped reversing contact 14 to the second fixed contacts 32 and 33, permitting the passage of current through the terminals 34a, 34b and 35, 36.
Turning next to FIG. 4, a description will be given of how tactile push button switches are used in practice. The switch 20 now in practical use usually has outer dimensions of a 5 mm width, a 5 mm depth and a 3 mm thickness at the largest. A number of such miniature switches 20 are fixedly mounted in a matrix form on the surface of a wiring board of a keyboard or like electric equipment and received in its housing. Reference numeral 17 denotes a top panel of the housing in which to receive the wiring board 16 with a number of tactile push button switches 20 fixedly mounted thereon; 71 denotes an annular shoulder defining or surrounding an aperture in the top panel 17 of the housing; 72 denotes an operating key loosely fitted in the aperture; 73 denotes an operating rod planted on the underside of the operating key 72; and 74 denotes a flange extending integrally from the outer peripheral edge of the operating key 72 at the lower end thereof.
In the normal state during which period the operating key 72 is not being pressed, it is pushed up by the push button 15 of the switch 20 and held at its raised position where the flange 74 is urged into engagement with the annular shoulder 71 around the aperture. By pressing the operating key 72, the push button switch 20 can be put in operation through the operating rod 73 and the push button 15.
Generally speaking, it is impossible to provide a completely watertight structure between the annular shoulder 71 of the top panel 17 surrounding its aperture and the flange of the operating key 72. Waterdrops permeate into the housing from between the marginal edge of the annular shoulder 71 and the flange 74 of the operating key 72 not only when they are disengaged from each other by the depression of the operating key 72 but also when they are engaged with each other. Incidentally, the switch 20 has its switch case 11, switch cover 12 and terminals 34 and 35 integrally molded into a completely watertight one-piece structure; however, the switch 20 is not watertight between the underside of the switch cover 12 and the flange 53 of the push button 15. When such a switch 20 is used, for example, in various portable remote controllers or portable telephones, care should be taken not to admit waterdrops into the opening 12A of the switch cover 12 over the entire circumference thereof. That is, waterdrops, once admitted into the housing of a keyboard or similar electrical equipment, are likely to stream into the switch 20 from between the underside of the switch cover 12 and the flange 53 of the push button 15.